The present invention relates to an ignition device for an internal combustion engine and, particularly, to an ignition device having a stop switch.
FIG. 8 is an example of a circuit diagram of a conventional ignition device which is also disclosed in Japanese Patent Publication No. 24565/1973. In FIG. 8, reference numeral 1 depicts an ignition signal generating circuit which includes a signal generator 11 for generating an alterating ignition timing signal in synchronism with an engine rotation, a diode 12, a resistor 13 and a capacitor 14. An ignition circuit 2 includes resistors 21, 23, 26 and 27, transistors 24, 28 and 29, a diode 25, an ignition coil 3 and a control circuit 4. The control circuit 4 is constituted by diodes 41 and 47, a capacitor 42, resistors 43 to 45 and a transistor 46. Reference numeral 5 depicts a battery.
When there is no ignition signal produced by the ignition signal generating circuit 1, the transistor 24 is made conductive by the resistors 21 and 22, which biases the diode 25 in reverse direction. Therefore, the transistor 28 is turned off and the transistor 29 is turned on to flow a current through the ignition coil 3. Thereafter, when an ignition signal is produced by the signal generator 11 due to an engine rotation, the signal is rectified by the diode 12 and supplied to a base of the transistor 24 to turn it off. Since the diode 25 is biased forwardly by the transistor 25 turned off, a base current is supplied through the resistor 23 to the transistor 28 to turn it on and turn the transistor 29 off. Therefore, a high voltage is induced in a secondary coil side of the ignition coil 3, by which the engine is ignited.
The diode 41 of the control circuit 4 is biased forwardly when the transistor 24 is turned off, so that the capacitor 42 is charged. Upon the charge of the capacitor 42, a base current of the transistor 46 is supplied to turn it on. Since the diode 47 is reverse-biased by the turned-on transistor 46, the control circuit 4 is isolated from the transistor 28, i.e., the ignition circuit 2. When the transistor 24 is turned on under this condition, the diode 41 is reverse-biased and the capacitor 42 discharges through the resistors 43 and 44. By setting the discharge time constant longer than the minimum ignition interval of the engine, it is possible to hold the transistor 46 conductive during the engine rotation and thus the control circuit 4 is isolated from the ignition circuit 2 during the engine rotation.
When the engine is stopped and the transistor 24 is turned on, the transistor 28 is turned off and the transistor 29 is turned on to allow a current to flow through the ignition coil 3. However, since the transistor 46 is turned off when the discharge of the capacitor 42 of the control circuit 4 is completed, a base current is supplied through the resistor 45 and the diode 47 to the transistor 28 to turn it on and turn the transistor 29 off, so that the current of the ignition coil 3 is cut off.
FIG. 9 shows another example of a conventional device and FIG. 10 shows waveforms at various points of the device shown in FIG. 9. In FIGS. 9 and 10, the circuit portion 4 of the example shown in FIG. 8 is substituted by a CPU 50. Other portions of the example in FIG. 9 are substantially the same as those shown in FIG. 8 and depicted by the same reference numerals, respectively.
An ignition control circuit 4, i.e., the CPU, has a reset terminal 50c connected to a power circuit 51 of a power supply 5 and is reset to start an operation from an initial condition when a key switch is operated, an output voltage of the power circuit 51 is lowered or an abnormal condition occurs. FIG. 10 shows this condition. As shown by a waveform (a) in FIG. 10, when the output voltage Vcc of the power supply circuit 51 is lowered below a predetermined level due to a voltage drop at a start of a starter motor, the power supply circuit 51 produces a reset signal whose level in a period t.sub.1 -t.sub.2 as shown by (b) in FIG. 10. In order to avoid a current flow through the ignition coil 3 at a start time of the ignition device, the ignition control circuit 4, i.e., the CPU 50 is designed to provide a H level signal at an output terminal C at t.sub.2, as shown by (c) and (e) in FIG. 10. With the H signal at the terminal C, the transistor 29' is turned off and the ignition control operation is started at the time when the current of the ignition coil 3.
Since the potential level at the terminal C is H at the start of the operation of the ignition device, an output impedance of the ignition control circuit 4 is high when the reset signal is supplied thereto (the period t.sub.1 -t.sub.2), the output potential thereof is always L level regardless of the potential at the terminal C, as shown by (c) and (e) in FIG. 10. Since, therefore, the transistor 29' is on during the period t.sub.1 -t.sub.2 and the ignition coil 3 is supplied with current, the ignition coil current is cut off at the start time (t.sub.1) of the ignition device so that a spark discharge also occurs, as shown by (d) and (f) in FIG. 10.
As mentioned above, in either of the conventional devices, a persistent current supply to the ignition coil 3 when the engine is not operating is prevented by the control circuit 4. In such case, however, since the current supply to the ignition coil 3 is performed in the same manner as in the period of ignition, a high voltage is induced in the secondary coil of the ignition coil 3 when the current supply is stopped and thus spark occurs. Therefore, depending upon a condition of the engine at a time when it is stopped, there is a possibility of damaging the engine due to the ignition of fuel which is left unburnt in a cylinder of the warmed up engine when the device is reset, even at an unsuitable crank position.
In order to prevent such phenomenon from occuring, an internal combustion engine ignition device having an emergency stop switch has been developed. In such device, a current supply to an ignition coil is usually stopped by an actuation of the stop switch. However, since such actuation causes a high voltage in a secondary side of the ignition coil to be induced as in the same manner mentioned above, the problem is still left as it is.